The circadian clock plays a fundamental role in the regulation and timing of human sleep. Not surprisingly, desynchrony of the circadian clock with the timing of the sleep/wake cycle underlies a significant percentage of sleep disorders including shift-work related dysomnia and advanced and delayed sleep phase syndromes. The environmental synchronizing agent for all circadian clocks is daylight, and phase response curves to single pulse application of bright light have been fully characterized for many organisms. Bright light treatment is therefore an ideal candidate for resynchronizing the circadian clock with the sleep/wake cycle, and recent studies have supported this hypothesis. However, human studies using single bright light pulses have been inadequate to unequivocally determine the phase response curve, specifically in the determination of the exact timing, the precise amplitude, and the temporal stability of the phase shifts. The purpose of this study is to fully characterize the human phase response curve to single pulses of bright light. Subjects will carefully have their initial circadian phase assayed using core body temperature measurement in a constant routine protocol designed to optimize this assessment. Following a calibrated exposure to a single 5 hour pulse of bright light at a selected circadian phase (ultimately, pulses will be judiciously applied over all circadian phases), the phase of the circadian rhythm will be assayed by 2 subsequent constant routine protocols to determine temporal stability of the phase shifts. These data are critical for a complete understanding of photic entrainment of the human circadian clock and for the design of treatments to ameliorate circadian sleep disorders.